Qubit-qubit interaction

Fig. Schematic overview of a CNOT operation between two qubits named control and target, by 1) exciting the control, 2) performing a NOT operation on the target, 3) de-exciting the control. If the control is initially in state |0〉, as in the figure, the resonance frequency of the target shifts, due to the dipole blockade effect, and the NOT operation is not performed. If the control instead starts in |1〉 it is never excited and the NOT operation is performed on the target qubit.

In addition to initializing qubits and performing single qubit gate operations, a coherent two-qubit interaction scheme is needed as well. For rare-earth-doped systems it has been suggested to use the dipole blockade effect to achieve such gates, i.e., using the difference of the permanent static dipole moment between the ground and excited states in order to shift one qubit out of resonance with the laser based on another qubit’s excitation. This effect requires spatially close ions since the induced frequency shift, in the dipole approximation, scales as 1/r3, where r is the distance between the qubits.

Presently, experiments are performed in order to prepare two ensemble qubits in Pr3+:Y2SiO5 so that the qubits only contain ions that are spatially close enough for the dipole blockade effect to work. So far, proof-of-principle experiments have been performed, but no full two-qubit gate has been implemented yet.